Question

I have an JPA entity like this:

@Entity
@Table(name = "category")
public class Category implements Serializable {
    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Basic(optional = false)
    @Column(name = "id")
    private Integer id;

    @Basic(optional = false)
    @Column(name = "name")
    private String name;

    @OneToMany(cascade = CascadeType.ALL, mappedBy = "category")
    private Collection<ItemCategory> itemCategoryCollection;

    //...
}

Use Mysql as the underlying database. “name” is designed as a unique key. Use Hibernate as JPA provider.

The problem with using merge method is that because pk is generated by db, so if the record already exist (the name is already there) then Hibernate will trying inserting it to db and I will get an unique key constrain violation exception and not doing the update . Does any one have a good practice to handle that? Thank you!

P.S: my workaround is like this:

public void save(Category entity) {

    Category existingEntity = this.find(entity.getName());
    if (existingEntity == null) {
       em.persist(entity);
       //code to commit ...
    } else {
        entity.setId(existingEntity.getId());
        em.merge(entity);
        //code to commit ...
    }
}

public Category find(String categoryName) {
    try {
        return (Category) getEm().createNamedQuery("Category.findByName").
                setParameter("name", categoryName).getSingleResult();
    } catch (NoResultException e) {
        return null;

    }
}

Answer 1

How to use em.merge() to insert OR update for jpa entities if primary key is generated by database?

Whether you’re using generated identifiers or not is IMO irrelevant. The problem here is that you want to implement an “upsert” on some unique key other than the PK and JPA doesn’t really provide support for that (merge relies on database identity).

So you have AFAIK 2 options.

Either perform an INSERT first and implement some retry mechanism in case of failure because of a unique constraint violation and then find and update the existing record (using a new entity manager).

Or, perform a SELECT first and then insert or update depending on the outcome of the SELECT (this is what you did). This works but is not 100% guaranteed as you can have a race condition between two concurrent threads (they might not find a record for a given categoryName and try to insert in parallel; the slowest thread will fail). If this is unlikely, it might be an acceptable solution.

Update: There might be a 3rd bonus option if you don’t mind using a MySQL proprietary feature, see 12.2.5.3. INSERT … ON DUPLICATE KEY UPDATE Syntax. Never tested with JPA though.